1. Field of the Invention
The present invention relates to granules suitable for manufacturing friction material used for brakes and clutches incorporated into various vehicles and industrial machines. Also, the present invention relates to a method of manufacturing the granules.
2. Description of the Related Art
Friction material widely used for brake pads, brake linings and clutch facings incorporated into various vehicles and industrial machines is manufactured as follows. Starting raw material containing raw material fiber such as heat-resistant organic fiber, inorganic fiber and metal fiber, and also containing raw material powder such as inorganic and organic filler, friction modifier and thermosetting resin binder is formed at a room temperature by a predetermined pressure (preforming) and then heat-formed at a predetermined temperature. Then it is cured (after-cure) and finished.
In the process in which the starting raw material is mixed and measured and then the measured material is charged into a die for heat-forming, powder (mixture) is handled, the shape of which is changeable. In many cases, raw material powder comprises fine particles, the diameters of which are several tens .mu.m, for the reasons of ensuring the frictional characteristic of the frictional material. Therefore, each process tends to be dusty when the material is handled. Accordingly, it is necessary to take countermeasure to prevent dust from rising, for example, it is necessary to arrange a shielding plate in each process. When the countermeasure is taken as described above, the manufacturing cost is increased. Further, the raw material powder is composed of fine particles, the specific surface of which is large. Therefore, the raw material powder tends to adhere onto an inner surface of the forming die. Accordingly, it becomes difficult to charge the starting raw material into the forming die uniformly.
In order to solve the above problems, the starting raw material is granulated. That is, various raw material powder and raw material fiber, which are the starting raw material, are rolled while binder is being added to them, so that granules, the diameter of which are 2 to 5 mm, can be obtained, and these granules are used as material for manufacturing friction material.
When raw material powder is granulated, it is possible to solve the above problem of dust which rises in the process of handling and it is also possible to solve the above problem of adhesion of raw material powder onto the inner surfaces of the forming die.
Raw material fiber is originally added for enhancing the mechanical strength of friction material when the raw material fiber is dispersed entirely and uniformly and entangled with each other. However, according to a method by which material is granulated, raw material fiber is involved in the granules and divided for each granule. Therefore, it is impossible for the raw material fiber to get entangled with each other. As a result, the reinforcement effect, which is the most important, is decreased, and it becomes impossible to provide the reinforcement effect in some cases.
Usually, granules are substantially spherical. When granules charged into a forming die are heat-formed at a predetermined temperature and pressure, it is difficult for each granule to be broken in the initial stage of heat-forming. Accordingly, there is a possibility that raw material of the friction material contained in granules is not sufficiently mixed with each other. As a result, uniformity of the friction material is deteriorated.